1. Field of the Invention
The present invention relates generally to the field of particulate and gas collectors and more particularly to a space efficient, hybrid collector incorporating electrostatic collectors and precipitators as well as particulate filters. The present invention is particularly attractive for air purification, exhaust after-treatment for vehicles, gas turbines and also applications such as high-tech, surgical and others that require the capture of sub-micron and nano-particles in both intake and exhaust air. Worldwide interest in gas turbine emissions and the enactment of Federal and State regulations in the United States have resulted in a need for an efficient means to control gas turbine exhaust emissions. The pollutants most generally of concern are CO, NOx, SOx, unburned hydrocarbons, soot. The present invention deals with such pollutants as unburned carbon, soot and other particulate emissions as well as pollutant gases. Additionally, the problem of purifying intake air for the combustion turbines is a matter of great importance with respect to the equipment maintenance and longevity by preventing the compression parts from being damaged by the high energy particulate impacting the rotating compression wheels.
Ultrahigh collection efficiency (>99.9999%) of submicron and nano particulate is of extreme interest in clean rooms like those used in semiconductor manufacture and the like. There is also considerable interest in killing bacteria and other organisms as well as removing particulate matter for air purifiers that might be used in surgical suits, or other applications where micro-organisms can be harmful.
2. Description of the Prior Art
Gas streams often carry particulate material. In many instances, it is desirable to remove some or all of the particulate material from a gas flow stream. Filters have been made in the past in a variety of configurations and designs, operating by mechanical or electrical entrapment of particles contained in the air stream. For example, a nonwoven or woven fibrous or filamentary web may by provided across the air flow for traversal thereby to trap dust particles in the greater-then-micron range. As is well known, such filters operate by mechanical interception of the particles, i.e. having a pore size which governs the size of particles which can be intercepted by a filter. Smaller particles traverse the filter and attempts to reduce the pore size lead only to high pressure drop and increasing impediment to has flow.
In order to achieve ultra high filtration efficiency, the prior art teaches that a very fine filter should be used (such as the type of filters called High Efficiency Particulate or HEPA filters). One of the major disadvantages of this prior art is the use of considerable energy needed to move air through the filter media. It would be a significant advantage to be able to achieve ultra high filtering efficiency without having to use such small grid filters.
Electrostatic precipitators have been proposed to alleviate this problem, the gas stream being passed through channels defined by oppositely charged plates, wires or screens, the latter representing collector electrodes which lie in arrays parallel to the direction of flow of the gas. The gas stream is subjected to a charging field, i.e., a corona discharge, and charged particulates are accumulated by absorption or dipole association with the dust particles in the gas stream. The electrically charges particles then transfer charge to the collecting electrode to this they adhere.
Some air purifiers pass an inlet air stream over one or more ionizing wires or an ionizing wire grid. The ionizing wires impart an electrical charge to the air flow, creating charged molecules known as ions, some of which eventually cling to airborne particles. In the case of air ionizers, the charged air is released as treated air. More sophisticated types of air purifiers include an electronic precipitator. Fleisher in U.S. Pat. No. 7,300,499 teaches the use of a high voltage electrostatic ionic air grid and precipitator plates for the removal of particulates together with ultraviolet light to provide germicidal properties. Kaura in U.S. Pat. No. 6,623,544 teaches treating air with mechanical filtration of particles greater than 2 micron, ionizing with energetic ions, and finally ultraviolet light to kill bacteria. Gillingham et al. teach the use of barrier media containing deposits of fine fibers in U.S. Pat. No. 7,270,692. Sikora in U.S. Pat. No. 5,433,772 teaches the use of a high voltage electrostatic filter with alternating oppositely charged plates with an ionizing wire affixed to alternate plates to remove particulates. U.S. Pat. Nos. 7,300,499, 6,623,544, 7,270,692 and 5,433,772 are hereby incorporated by reference.
Exhaust gas after-treatment in vehicles is well known in the art and is commonly used to meet emission requirements. Current after-treatment is used to remove unwanted nitrogen and sulfur compounds as well as particulate matter. In diesel systems electrostatic collectors have been used to remove suspended particulate matter including oil droplets from the blow-by gas, for example, so that blow-by gas can be returned to the atmosphere or to the fresh air intake side of the diesel engine for further combustion. Also, there are numerous applications that require ultra-filtering including the removal of sub-micron and nano-particles.
For example, the use of fossil fuel in gas turbine engines results in the combustion products consisting of carbon dioxide, water vapor, oxides of nitrogen, carbon monoxide, unburned hydrocarbons, oxides of sulfur and particulates. Of these products, carbon dioxide and water vapor are generally not considered objectionable at least as pollutants. In most applications, governmental imposed regulations are further restricting the remainder of the constituents emitted in the exhaust gases. The majority of the products of combustion emitted in the exhaust can be controlled by design modifications, cleanup of exhaust gases and/or regulating the quality of fuel used. For example, sulfur oxides could be controlled by the selection of fuels that are low in total sulfur. This leaves nitrogen oxides, carbon monoxide and unburned hydrocarbons as the emissions of primary concern in the exhaust gases emitted from the gas turbine or an automotive engine. Particulates in the engine exhaust have been controlled either by design modifications to the combustors and fuel injectors or by removing them by traps and filters.
For gas turbines, combined cycle gas turbines are generally better candidates for emissions control than simple cycle units. The main challenge in a combined cycle system is to find enough space to house the emissions control unit within the Heat Recovery Steam Generator (HRSG) in the proper temperature regime. Depending on the exhaust temperatures, simple cycle gas turbines present a somewhat more complicated challenge The lower exhaust temperature of some mature frame gas turbines which is usually well below 450° C. (842° F.) is within the operating capability of conventional technologies and materials. In a simple cycle gas turbine configuration, the emissions control system is normally located immediately downstream of the gas turbine and requires an expansion from the gas turbine outlet exhaust duct to the emissions control system.
Electrostatic collectors and particulate filters are also known in the art. In some of my previous patents, I teach systems containing these components. (Krigmont—U.S. Pat. Nos. 6,932,857; 6,524,369; 5,547,493) used to clean flue gas in power plants. U.S. Pat. Nos. 6,932,857, 6,524,369 and 5,547,493 are hereby incorporated by reference. In addition, Chang in U.S. Pat. No. 7,267,712 teaches picking up charged particles in an electric field with a filter, while others (U.S. 2005/925170, US2006/524369, U.S. 2005/322550 and U.S. 2005/492557) teach various electrostatic air cleaners.
Prior art systems are many times large and expensive and do not necessarily provide the type of filtering needed for vehicle or portable applications. It would be advantageous to have a compact, space-efficient hybrid collector that uses corona discharge to charge particles and partially collect them and the combination of porous surfaces and uniform electric fields to collect the remaining particles for use in an automotive or other vehicle exhaust emissions control system, combustion turbines or for any other space-restricted or portable use including high technology uses such as surgery and semi-conductor manufacture where it is important to trap sub-micron and even nano-particles. It would also be advantageous to optionally use barrier filters known in the art to convert hazardous compounds into more benign substances.